CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,122.6
Allowance for credit losses	164.7	$ 140.0
Accounts receivable and unbilled revenues, net	957.9
Total accounts receivable, net - past due greater than 90 days	$ 139.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	91.70%
Amount of net accounts receivable with regulatory protections	$ 530.6
Percent of net accounts receivable with regulatory protections	55.40%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,112.2
Allowance for credit losses	164.6	139.9
Accounts receivable and unbilled revenues, net	947.6
Total accounts receivable, net - past due greater than 90 days	$ 139.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	91.70%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 771.3
Allowance for credit losses	77.9	59.9
Accounts receivable and unbilled revenues, net	693.4
Total accounts receivable, net - past due greater than 90 days	$ 72.6
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	93.80%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 298.9
Allowance for credit losses	82.7	75.9
Accounts receivable and unbilled revenues, net	216.2
Total accounts receivable, net - past due greater than 90 days	$ 59.3
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	100.00%
Other States | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 42.0
Allowance for credit losses	4.0	4.1
Accounts receivable and unbilled revenues, net	38.0
Total accounts receivable, net - past due greater than 90 days	$ 7.1
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%
Non-Utility Energy Infrastructure
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 6.3
Allowance for credit losses	0.0
Accounts receivable and unbilled revenues, net	6.3
Total accounts receivable, net - past due greater than 90 days	$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%
Corporate and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 4.1
Allowance for credit losses	0.1	$ 0.1
Accounts receivable and unbilled revenues, net	4.0
Total accounts receivable, net - past due greater than 90 days	$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%
COVID-19
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in allowance for credit loss	$ 3.4